Titanium nitride (TiN) coatings are of interest for wear, erosion and corrosion applications because of their high hardness and good chemical stability. Physical vapor deposition (PVD) and chemical vapor deposition (CVD) processes have been successfully used to deposit thin TiN coatings on metal and alloy components for a variety of engineering applications. The benefits from their excellent wear characteristics are well recognized on TiN coated cutting and forming tools, and their good erosion protection from solid particle impact is evident on TiN coated compressor blades in turbomachines. For corrosion protection, however, relatively little success has been reported and then only in a mildly corrosive environment.
The corrosion behavior of TiN coatings depends on their chemistry, microstructure and thickness. A nearly stoichiometric TiN and a N-rich TiN were found to have good corrosion resistance. It was reported in the prior art that a fully dense TiN coating with fine equiaxial grains is superior to many engineering materials in resistance to various corrosive environments. It was also reported that a coating with a columnar structure generally has poor corrosion resistance due to micropores at the boundaries of the columnar grains. In essence, pinholes and micropores in the coating are most detrimental to its corrosion properties. This is because TiN is electrochemically more noble than most engineering materials. Pinholes in the coating may accelerate corrosion in the underlying metal by pitting, crevice or galvanic corrosion mechanisms, particularly in a chloride-containing environment. The corrosion resistance of TiN was also shown to increase with increasing coating thickness due to reducing the occurrence of possible pinholes penetrating through the coating. Typically, it requires a thick coating (20 .mu.m) to provide full protection of the base metal from the corrosive media. However, it is difficult to deposit such a thick coating uniformly on engineering components due to their complex configuration and the relatively high intrinsic compressive stress in the coating. Clearly, pinholes prevent the TiN coating from being successfully used for a combined corrosion and wear/erosion application. In addition, pinholes are an inherent structural defect in PVD and CVD type TiN coatings.
Many approaches have been utilized to improve the corrosion resistance of TiN coatings by reducing pinhole defects through the use of a metallic intermediate layer or multilayer deposits. U.S. Pat. No. 4,226,082 discloses that a Ti+TiN coating with a Ti-rich intermediate layer adjacent to the base metal provides good properties for an ornamental application in a relatively mild wear and corrosive environment. It was reported in an article by Massiani et al. in Surface Coating Technologies 45 (1991) 115, that a TiN coating with a Ti intermediate layer improves corrosion protection over a single layer TiN coating on AISI 430 and AISI 304 stainless steel and Fe substrates in H.sub.2 SO.sub.4 and NaCl solutions. Japanese Patent No. 8,064,380 discloses that a Ti layer laminating alternately with TiN can inhibit the formation of pinholes passing through the coating layer to protect the coated article from localized corrosion attack.
An object of the present invention is to provide a multilayer coated substrate having good wear, erosion and corrosion resistance characteristics which is effective in reducing interconnected pinholes or pores through the coating to the substrate.
Another object of the present invention is to provide a multilayer coated substrate comprising a first zone comprising at least one layer of a titanium nitrogen-containing layer deposited between the surface of the substrate and a second zone that comprises at least two layers of a titanium nitride-containing compound.
Another object of the invention is to provide a method for producing a multilayered coating of a titanium nitrogen-containing compound that can effectively eliminate galvanic corrosion attack within the coating layers.
The above and further objects and advantages of this invention will become apparent upon consideration of the following description.